4. The vegetable oils and animal fats, capable of saponification, are very numerous; but those most commonly employed in the manufacture of the soaps of commerce, are olive-oil, whale-oil, tallow, lard, palm-oil, and rosin; and the alkalies with which these are most frequently combined, are soda, the ley of ashes, or its residuum, potash.
5. Soda is sometimes called the mineral alkali; because it is found, in some parts of the world, in the earth. It was known to the ancients, at a very early period, under the denomination of natron. It received this appellation from the lakes of Natron, in Egypt, from the waters of which it was produced by evaporation, during the summer season.
6. The soda of commerce is now chiefly obtained from the salsola, a genus of plants which grows on the sea-shore. In Spain, the plant from which soda is obtained is denominated barilla; hence, the substance produced from it by incineration has received the same appellation. The ashes of a sea-weed which grows on the coasts of Scotland and Ireland, is called kelp. In Europe, barilla and kelp are more extensively employed in the manufacture of soap than any other alkaline substances; but, in this country, where wood is so much used for fuel, common ashes are generally preferred.
7. The process of making the ordinary brown or yellow soap, from wood-ashes, is conducted in the following manner. The alkali is first obtained in a state of solution in water, by leeching the ashes as described in [page 26], and then poured, in a weak state, into a copper or iron caldron, having a large wooden tub carefully affixed to the top of it.
8. When the ley has been properly heated, the tallow, either in a tried state or in the suet, is gradually added. More ley, of greater concentration, is poured in; and the ingredients are moderately boiled for several hours; while a person, as represented in the preceding cut, aids their chemical union by agitating them with a wooden spatula.
9. After a quantity of rosin has been added, and properly incorporated with the other materials, the fire is withdrawn until the next morning, when it is again raised; then, with the view of forming the paste into hard soap, a quantity of muriate of soda (common salt) is added. The muriatic acid of this substance, uniting with the potash, forms with it muriate of potash, which dissolves in the water, while the soda combines with the tallow and rosin. Hard soap, therefore, contains no potash; although this alkali is generally employed during the early part of the process of making it.
10. After the addition of the muriate of soda, the boiling and stirring are continued two or three hours, when the fire is withdrawn, and the contents of the caldron are suffered to be at rest. When the soap has completely separated from the watery part and extraneous matters, it is laded into another caldron, again diluted with strong ley, and heated. The paste having been brought to a proper consistence, more common salt is added as before, and for the same purposes.
11. The chemical part of the process having been thus completed, the soap is laded into single wooden boxes, or into one or more composed of several distinct frames, which can be removed separately from the soap, after it has become solid enough to stand without such support. The soap is cut into bars, of nearly a uniform size, by means of a small brass wire.
12. Manufacturers of soap have contrived various methods of adulterating this article, or of adding ingredients which increase its weight, without adding to its value. The most common means employed for this purpose is water, which may be added, in some cases, in considerable quantities, without greatly diminishing the consistence of the soap.
13. This fraud may be detected by letting the soap lie for some time exposed to the atmosphere. The water will thus be evaporated, and its quantity can be known by weighing the soap, after its loss of the superfluous liquid. To prevent evaporation, while the soap remains on hand, it is said, that some dealers keep it in saturated solutions of common salt. Another method of adulteration is found in the use of pulverized lime, gypsum, or pipe-clay. These substances, however, can be easily detected by means of a solution in alcohol, which precipitates them.